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SOHO SEES RIGHT THROUGH THE SUN TO FIND STORMY REGIONS ON THE OTHER
SIDE

A week's advance warning of potential bad weather in space is
now possible thanks to the Solar and Heliospheric Observatory
(SOHO) spacecraft. With a technique that uses ripples on the Sun's
visible surface to probe its interior, SOHO scientists have, for
the first time, imaged solar storm regions on the far side of the
Sun, the side facing away from the Earth.

Like the unanticipated arrival of hurricanes before the
advent of weather satellites, a group of previously hidden solar
storm regions can rotate suddenly into view as the Sun turns,
blazing away with explosive eruptions. The new technique, which
uses the Michelson Doppler Imager (MDI) instrument on SOHO, gives
a warning by creating a window to the far side of the Sun.

"We've known for ten years that in theory we could make the
Sun transparent all the way to the far side," according to Dr.
Charles Lindsey of Solar Physics Research Corp., Tucson, AZ, and
Dr. Douglas Braun of NorthWest Research Associates, Boulder, CO,
authors of a paper describing the research to be published in the
journal Science on March 10. "But we needed observations of
exceptional quality. In the end we got them, from MDI on SOHO."

Storm areas on the Sun, called active regions, are much
larger than the Earth and consist of strong magnetic fields on the
Sun's surface. Active regions produce explosions, called flares,
and eruptions of plasma (hot, electrically charged gas), called
coronal mass ejections. The radiation and plasma from these events
sweep past the Earth, sometimes affecting spacecraft, power
systems and disrupting radio communications. Understanding and
forecasting solar eruptions and their consequences is a relatively
new science called space weather.

For more than 100 years, scientists have been aware that
sunspots (groups of dark areas on the Sun's visible face) are
often the scene of flares and other eruptions. Now they watch the
Sun more closely than ever, because modern systems are much more
vulnerable to solar disturbances than old technology. The experts
can still be taken by surprise because the Sun rotates, bringing
the effects of hidden active regions to bear on Earth. With a far-
side preview of sunspots, nasty surprises for the space weather
experts may now be avoidable.

Ripples on the Sun's surface used to image the interior are
caused by sound waves reverberating through the Sun. Analysis of
solar sound waves is the science of helioseismology, and it opened
the Sun's gaseous interior to investigation in much the same way
as seismologists learned to explore the Earth's rocky interior
with earthquake waves.

The MDI instrument is the most elaborate of three
helioseismic instruments on SOHO. It measures rhythmic motions at
a million points across the Sun's visible surface.

Computers can interpret the motions in terms of sound waves
travelling through the Sun. The waves are affected by the various
layers of gas and different motions that they encounter. The MDI
has already revealed many unknown features of the solar interior,
including hidden jet streams circling the Sun's poles.

The technique of helioseismic holography used by Lindsey and
Braun examines a wide ring of sound waves that emanate from a
small region on the far side and reach the near side by rebounding
internally from the solar surface. An active region reveals itself
because it possesses very strong magnetic fields that speed up the
sound waves. Waves that pass through an active region have a
round-trip travel time about 12 seconds shorter than the average
of six hours. The difference becomes evident when sound waves
shuttling back and forth get out of step with one another.

MDI data for March 28-29, 1998, revealed on the far side a
sunspot group that was not plainly visible on the near side until
10 days later. Observations for 24 hours were more than sufficient
to detect the sunspots, which means that routine monitoring is a
realistic possibility.

SOHO is a cooperative project between the European Space
Agency (ESA) and NASA. The spacecraft was built in Europe for ESA
and equipped with instruments by teams of scientists in Europe and
the USA. The far-side helioseismology research was funded by the
National Science Foundation and NASA.